The invention relates to a method of energizing a polyphase motor by means of a plurality of coils, each coil intended to be excited by means of a control signal.
Polyphase motors are customarily used in many applications, particularly within information-storage systems including a disc at the surface of which the information is stored, and at least one read and/or write head for scanning the surface of said disc, which disc is driven to rotation by the polyphase motor.
The information can for example be engraved on the disc, allowing it to be optically read, or the information may consist of local magnetic fields, allowing it to be magnetically read, for example by means of a magneto-resistive rod. The hard disks used in the information industry are of the latter type.
The motors used in said latter type of applications generally are three-phase motors, i.e. they are provided with three coils which are to be excited by three currents, forming the control signals, which are successively shifted through 2.xcfx80/3, which is achieved by subjecting the coils to three voltages, which are also successively shifted through 2.xcfx80/3 by means of a power supply device. A voltage is supplied to each coil by a power stage comprising two transistors, which are arranged in series between two supply terminals, i.e. a positive supply terminal and ground, an intermediate node between said transistors being connected to the coil.
In accordance with the currently used power supply methods, the supply voltage applied to each of the coils results from a particular control of the switching of the transistors included in the associated power stage. However, owing to large parasitic capacitances due to the large size of the power transistors, the switchings of these transistors cause losses which are not inconsiderable and hence have an adverse effect on the energy efficiency of the power supply device of the motor.
One of the objects of the invention is to improve the efficiency by providing a power supply method requiring a smaller number of switching operations of the transistors included in the power stages as compared to the known power supply methods used in polyphase motors.
To achieve this, a power supply method as described in the opening paragraph comprises the following stages:
identifying the control signal having the largest absolute value, and
connecting the corresponding coil to a reference-potential terminal.
The invention enables the number of switching operations of the power transistors, which are necessary to energize a three-phase motor, to be reduced by one third, because each coil is connected to a reference-potential terminal during one third of the period of its control signal. In more general terms, the invention enables the number of switching operations of power transistors, necessary to energize a motor comprising N coils, to be reduced by 1/N.
In a variant of the invention, a power supply method as described in the opening paragraph includes the following stages:
identifying the control signal having the largest instantaneous value, and
connecting the corresponding coil to a reference-potential terminal.
In this variant of the invention, the coils are alternately connected to the same reference-potential terminal, for example the positive supply terminal. By virtue thereof, the structure of the circuitry used to implement the invention can be simplified.
In another variant of the invention, a power supply method as described in the opening paragraph includes the following stages:
identifying the control signal having the smallest instantaneous value, and
connecting the corresponding coil to a reference-potential terminal.
In this variant of the invention, the coils are alternately connected to the same reference-potential terminal, for example ground. By virtue thereof, the structure of the circuitry used to implement the invention can be simplified.
In a particular embodiment in accordance with the invention, the identification step includes detection of a sign reversal of a control signal.
Such a detection enables a simple implementation of the identification step.
In an advantageous embodiment of the invention, each control signal is composed of a succession of discrete values which follow a pseudo-sine line as a function of time when the coil excited by said signal is not connected to a reference-potential terminal. This embodiment enables the value of the supply voltage on each one of the coils to be easily monitored, in digital form.
In a preferred embodiment of the invention, each discrete value is obtained by setting a duty cycle of a signal referred to as driving signal, which serves to control the conduction of two complementary transistors, which are arranged in series between two supply terminals, an intermediate node between said transistors being connected to one terminal of one of the coils.
By virtue of this embodiment, it is ensured that the control signals all follow a polyphase sinusoidal line, which corresponds to the line they would follow if they had to excite polyphase coils that were not alternately connected to one reference-potential terminal, as is the case in known power supply methods. As will be described later in this document, apart from the reduction of losses caused by the switching operations of the transistors included in the power stages, the invention enables the supply voltage of these power stages to be reduced without a modification of the mechanical energy supplied by the motor, or an increase of this mechanical energy at an equal supply voltage, which is tantamount to a substantial, advantageous increase of the energy efficiency of the power supply device of the motor.
In one of the possible embodiments, the invention relates to a device for supplying power to a polyphase motor comprising a plurality of coils, each of which is to be excited by means of a control signal, which device comprises:
control means enabling each coil to be alternately connected to a reference-potential terminal, and
identification means enabling identification of the coil which is to be connected to the reference-potential terminal.
As described hereinabove, the coil that will be identified by the identification means in order to be connected to a reference-potential terminal, can be the coil that is subjected to the control signal having the largest absolute value, the largest instantaneous value, or the lowest instantaneous value.
Though the invention can suitably be used in all types of systems where a polyphase motor is used, it can be implemented particularly advantageously in systems for storing information. Thus, the invention also relates to an apparatus for storing information, comprising:
a disc on the surface of which information is stored,
a polyphase motor intended to drive the disc to rotation,
at least one read/write head intended to scan the surface of the disc, and
a device for supplying power to the motor, as described hereinabove.
These and other aspects of the invention will be apparent from and elucidated with reference to the exemplary embodiment(s) described hereinafter.